1. Field of the Invention
The present invention relates to a fuel pump which supplies fuel to an internal combustion engine of a vehicle from a fuel tank.
2. Description of the Related Art
In a returnless type fuel supply system for an internal combustion engine, excessive fuel and fuel vapor generated due to high temperature of the engine do not return to the fuel tank and, therefore, volatile components of the fuel remain in the fuel supply system. In a fuel pump having an impeller, fuel vapor is apt to be generated in the pump passage formed around the impeller. When the engine is idling, the amount of fuel injected by the pump is small because the fuel pump is controlled by an electronic control unit (hereinafter referred to as ECU), and the fuel vapor can not be discharged from the pump passage sufficiently. If the vapor remains in the pump passage, liquid fuel is cut into pieces by bubbles of the vapor and the pump passage can not be pressurized properly. As a result the fuel pump can not supply fuel to the engine properly (e.g., it is sometimes said to be "vapor locked").
In a fuel supply system having a fuel return passage as disclosed in JP-B-3-61038, a vapor discharging port is formed. However, such vapor discharging port can not be applied to the fuel pump used in the returnless type fuel supply system because most of the vapor remains around the fuel inlet port and does not move to the vapor discharging port.